From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: From: Paul Gortmaker To: , CC: Will Deacon , Arnd Bergmann , Chris Mason , Ingo Molnar , Peter Zijlstra , Thomas Gleixner , Paul Gortmaker Subject: [v2.6.34-stable 70/77] mutex: Place lock in contended state after fastpath_lock failure Date: Tue, 8 Jan 2013 18:35:49 -0500 Message-ID: <1357688156-25387-71-git-send-email-paul.gortmaker@windriver.com> In-Reply-To: <1357688156-25387-1-git-send-email-paul.gortmaker@windriver.com> References: <1357688156-25387-1-git-send-email-paul.gortmaker@windriver.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: From: Will Deacon ------------------- This is a commit scheduled for the next v2.6.34 longterm release. http://git.kernel.org/?p=linux/kernel/git/paulg/longterm-queue-2.6.34.git If you see a problem with using this for longterm, please comment. ------------------- commit 0bce9c46bf3b15f485d82d7e81dabed6ebcc24b1 upstream. ARM recently moved to asm-generic/mutex-xchg.h for its mutex implementation after the previous implementation was found to be missing some crucial memory barriers. However, this has revealed some problems running hackbench on SMP platforms due to the way in which the MUTEX_SPIN_ON_OWNER code operates. The symptoms are that a bunch of hackbench tasks are left waiting on an unlocked mutex and therefore never get woken up to claim it. This boils down to the following sequence of events: Task A Task B Task C Lock value 0 1 1 lock() 0 2 lock() 0 3 spin(A) 0 4 unlock() 1 5 lock() 0 6 cmpxchg(1,0) 0 7 contended() -1 8 lock() 0 9 spin(C) 0 10 unlock() 1 11 cmpxchg(1,0) 0 12 unlock() 1 At this point, the lock is unlocked, but Task B is in an uninterruptible sleep with nobody to wake it up. This patch fixes the problem by ensuring we put the lock into the contended state if we fail to acquire it on the fastpath, ensuring that any blocked waiters are woken up when the mutex is released. Signed-off-by: Will Deacon Cc: Arnd Bergmann Cc: Chris Mason Cc: Ingo Molnar Cc: Reviewed-by: Nicolas Pitre Signed-off-by: Peter Zijlstra Link: http://lkml.kernel.org/n/tip-6e9lrw2avczr0617fzl5vqb8@git.kernel.org Signed-off-by: Thomas Gleixner Signed-off-by: Paul Gortmaker --- include/asm-generic/mutex-xchg.h | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h index 580a6d3..c04e0db 100644 --- a/include/asm-generic/mutex-xchg.h +++ b/include/asm-generic/mutex-xchg.h @@ -26,7 +26,13 @@ static inline void __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *)) { if (unlikely(atomic_xchg(count, 0) != 1)) - fail_fn(count); + /* + * We failed to acquire the lock, so mark it contended + * to ensure that any waiting tasks are woken up by the + * unlock slow path. + */ + if (likely(atomic_xchg(count, -1) != 1)) + fail_fn(count); } /** @@ -43,7 +49,8 @@ static inline int __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *)) { if (unlikely(atomic_xchg(count, 0) != 1)) - return fail_fn(count); + if (likely(atomic_xchg(count, -1) != 1)) + return fail_fn(count); return 0; } -- 1.7.12.1